In release 6 of the 3GPP WCDMA specifications, the support for high speed packet data in the uplink has been improved with a new uplink transport channel, Enhanced Dedicated Transport Channel (E-DCH). The E-DCH supports higher data rates, Node B HARQ with soft combining and a fast Node B scheduling.
With the new scheduling functionality, the Node B can limit the maximum rate that a UE may use by issuing “grants” to the UE. However, as the Node B is not aware of the detailed buffer status or the available power in the UE, the final rate selection resides in the UE. The system and functionality description for the Enhanced uplink describes the mechanism for the rate selection in the UE. Basically the UE should use the minimum rate selected among the issued grant, “system granted rate” or “UE evaluated maximum rate based on power consumptions”.
When the UE is limited by its own maximum Tx power, it has to evaluate which rate it can support (the UE does this evaluation all the time in order to find out whether it is power or grant limited) in order not to end up in a power shortage situation. With a power shortage situation is meant that the UE does not have enough power to transmit a selected transport format, E-TFC, with the intended power. This means that the UE has overestimated the rate, that can be supported and the transport block is transmitted with lower power than what is configured. As a consequence the probability that the transmission cannot be detect correctly is increased and additional transmissions may be needed.
When the UE starts to experience power shortage, the long term (and preferred) solution is to change the used E-TFC so that the available power is enough for a proper transmission. This action takes some TTIs to accomplish. During this execution time, the UE starts to temporarily reduce the power allocation to the Enhanced data channel (E-DPDCH).
The UE rate selection (E-TFC selection) is based on the power used for the control channel (DPCCH) together with power offset values for each existing transport format (E-TFC), i.e. βed. Based on the DPCCH power and βed's, the UE evaluates which E-TFC it can support. The intermediate temporal power reduction is carried out by reducing the βed factor on slot basis.
For the UE to request resources from the Node B(s), scheduling requests are transmitted in the uplink in the form of Scheduling Information (SI) and Happy Bit. The SI will be transmitted for the logical channels configured so that reporting has to be made, while the Happy Bit always should be included in the E-DPCCH, whenever the E-DPCCH is transmitted.
The SI is located at the end of the MAC-e PDU and is used to provide the serving Node B with a better view of the amount of system resources needed by the UE and the amount of resources the UE actually can make use of.
The UE includes the following in the Scheduling Information: ‘Logical channel ID of the highest priority channel with data in buffer (HLID, 4 bits)’; ‘Total E-DCH Buffer Status (TEBS, 5 bits), ‘Highest priority Logical channel Buffer Status’ (HLBS, 4 bits); and ‘UE Power Headroom (UPH, 5 bits)’, i.e. this field indicates the ratio of the maximum UE transmission power and the corresponding DPCCH code power.
The reported UE transmission power headroom measurement result shall be an estimate of the average value of the UE transmission power headroom over a 100 ms period.
According to current procedures, the SI will be reported to the Node B following some criterion. In the case where the UE is not allowed to transmit scheduled data (because it has no serving grant available or it has received an absolute grant preventing it from transmitting in any process) and it has scheduled data to send on a logical channel for which SI must be reported the serving E-DCH RLS in a MAC-e PDU:                Periodic reporting to protect against NACK-to-ACK misinterpretation;        Scheduling information could be sent alone, or with non-scheduled data, if such exist;        Scheduling information will also be triggered if higher priority data arrives in buffer.        
From a “not wasting system resources” point of view, the scenario where the UE is allowed to transmit scheduled data and has scheduled data to send on a logical channel for which SI must be reported but do not have enough power left, is important to handle.
Using today's solution for selecting when a SI (UPH information) should be triggered, there is no mechanism that takes the UE's power situation into consideration, nor it is possible for the UE to decide when to inform the system (Node B) about the current situation.
Not having this information, the Node B cannot have updated information on the local power demand (interference situation), and risks to waste system capacity. For example, during the temporary power reduction of the E-DPDCH, retransmission(s) will trigger repeated increases of the SIR-target through the Outer-Loop Power Control (OLPC), thus wasting capacity and end-user throughput. Additionally, the Node B might give grant to a UE that cannot make use of it.